1. Field of the Invention
The present invention relates to a production method for a bushing used in an endless track mounted to vehicles.
2. Description of the Related Art
An endless track adapted to be mounted to vehicles (for example, a bulldozer) includes, as shown in FIG. 1, a shoe 2, shoe connecting bolts 3, shoe nuts 4, links 5 and 6, bushings 7, dust seals 8, and pins 9 as one structural unit thereof.
The bushing 7 used for an endless track is shown in FIG. 2 in an enlarged manner. For the endless track bushing, abrasion resistance is required at an inside surface 7a, an outside surface 7b and portions 7c adjacent the surfaces 7a and 7b, and strength and toughness are required at the entire portions including a core portion 7d to endure a load imposed on the bushing.
To satisfy those requirements, the following production methods of an endless track bushing have been proposed:
(a) A production method as proposed in Japanese Patent Publication SHO 52-34806, wherein case hardening steel (JIS (Japanese Industrial Standard): SCM415), which is a low-carbon steel, is selected as the bushing material. The bushing material is carburized at portions near the surfaces thereof in the furnace and is cooled to an ambient temperature. Then, the bushing material is heated and quenched by oil, and then the bushing material is tempered. The required hardness for abrasion resistance at the surfaces is obtained through the carburizing and the heat treatment after the carburizing, and the required strength and toughness are obtained through the quenching and tempering. This method will be called a first related art hereinafter.
(b) A production method as proposed in Japanese Patent Publication HEI 1-259129 published Oct. 16, 1989 (Japanese Patent Application SHO 63-87338) and as shown in FIG. 3. In the method, a bushing material 10 of medium-carbon steel is carburized, and then the bushing material is cooled to an ambient temperature. Then, the bushing material is induction-heated and quenched beyond an outer carburized layer from an outside surface 10b thereof while the bushing material 10 is rotated about an axis 10a thereof so that an outer effective hardened layer having a hardness greater than a specified effective hardness is formed. Then, the bushing material is induction-heated and quenched beyond an inner carburized layer from an inside surface 10c thereof while the bushing material 10 is rotated about the axis 10a and the outside surface is cooled by coolant whereby an inner effective hardened layer having a hardness greater than the specified effective hardness is formed and a tempered layer having a hardness less than the specified effective hardness is formed between the inner and outer effective hardened layer. Finally, the bushing material is tempered at low temperatures. This method will be called a second related art hereinafter.
(c) A production method as proposed in Japanese Patent Application SHO 63-320420 (Japanese Publication HEI 2-169375 which corresponds to U.S. Pat. Application No. 07/454,004), wherein a bushing material of medium-carbon steel is carburized and then cooled to an ambient temperature. Then, the bushing material is induction-heated from an outside surface of the bushing material only so that an entire cross section of a wall of the bushing material is heated, and then the bushing material is rapidly cooled from both the inside surface and the outside surface of the bushing material. Lastly, the bushing material is tempered. In this method, the required surface hardness is obtained through the carburizing and the heat treatment executed after the carburizing, and the required toughness is obtained through a residual compressive stresses produced at the inside and outside surfaces of the bushing material utilizing the difference in carbon content between the surface portions and the core portion of the bushing material due to the carburizing. More particularly, it is known that the more the carbon content is, the more the steel grain is expanded when quenched. Since the inside and outside surface portions of the bushing material include more carbon than the core portion, the surface portions will be more expanded than the core portion when quenched. Then, when the bushing material is rapidly cooled from both the inside and outside surfaces, a residual compressive stress is produced in the surface portions. When the bushing receives a compressive load at opposite outside surfaces, the compressive load generates a tensile stress at the opposite portions of the inside surface. Since the tensile stress is cancelled by the residual compressive stress, the toughness of the bushing is increased. Thus, in this method, the carburizing is indispensable for both increasing the surface hardness and the toughness of the bushing. This method will be called a third related art hereinafter.
A high carbon content bushing material is not used in the prior production methods because the necessary carbon content difference between the surface portions of the bushings and the core portion of the bushings would not be achieved. Without this carbon content differential between the core and surface portions of the bushing material, it was thought that the necessary residual compressive stresses could not be formed. Thus, the use of high carbon steel without carburizing has not previously been realized.
However, the first related art takes a relatively long time period to carburize the bushing material and is relatively expensive, because the case hardening steel is a low-carbon steel. The second related art also requires carburizing which takes a yet long time period. Further, the second related art requires two quenchings, i.e., quenching from the outside surface and then quenching from the inside surface. Therefore, the heat treatment requires a relatively long time and is expensive. The third related art requires carburizing. As a result, the third related art requires a yet long time period and is yet expensive.